The present invention relates generally to radio communications systems, and more particularly to an improved method and apparatus for dynamically selecting one of a plurality of radio frequency signal transmitters for transmitting message signals from a primary station to remote stations of a data communications system.
In radio communications systems covering large geographical areas, the location of remote stations such as portable or mobile radios must be known with reasonable accuracy in order to provide good quality communications. In such communications systems, the geographical area may be divided up into a number of zones or cells, each of which is covered by at least one radio transmitter and radio receiver associated with a primary station. For establishing communications between the primary station and a remote station, knowledge of the remote station's location is necessary in order that the radio transmitter covering the zone in which the remote station is located may be selected at the primary station.
The problem of selecting the radio transmitter which covers the zone in which a remote station is located has been solved with a limited degree of success in several different ways. According to one technique, the radio receiver receiving the strongest RF signal from a selected remote station is used to define the location of that remote station. The primary station simply selects the radio transmitter covering the geographical area of the receiver receiving the strongest signal from the selected remote station.
According to another technique, each remote station is assigned to a specific geographical area. In other words, a remote station is permanently associated with the zone or zones covered by a specific radio transmitter. This technique works reasonably well as long as the remote station remains within the geographical area covered by the assigned radio transmitter. However, this technique is inadequate for communications systems where each remote station is free to move about throughout a very large geographical area, making it impossible to limit a remote station to the coverage area of a single radio transmitter.
According to yet another technique that is utilized in cellular radiotelephone systems, the remote station determines the zone in which it is located by selecting the radio transmitter having the largest signal strength. This technique requires that each radio transmitter have a different frequency, and that communications from the primary station to a selected remote station be sent in all zones in order for the remote station to make its choice known on demand. This technique is adequate for radio telephone systems where the average message length is much longer than the minimum message length, but is inadequate for data communications sytems where the average message length is not much larger than the minimum message length. Therefore, in order to provide good quality communications in data communications systems, it is necessary to have a reasonably accurate determination of the location of each remote station in the system.